The behavior of engineering structures under load is determined by many design parameters, and defining the optimum material for a specific application is often a tedious task and moreover has to deal with conflicting requirements. Among the commonly used engineering materials are metals, like steel alloys, titanium alloys, magnesium alloys, aluminum alloys; fiber-reinforced composites, like glass fiber composites, carbon fiber composites, and aramid composites; and hybrid materials, further defined below.
Fiber-reinforced composites offer considerable weight advantage over other preferred materials, such as metals. Generally, the weight savings are obtained at the sacrifice of other important material properties such as ductility, toughness, bearing strength, conductivity and cold forming capability. To overcome these deficiencies, new hybrid materials called fiber-metal laminates have been developed to combine the best attributes of metal and composites.
Fiber-metal laminates (also referred to as FML), such as those described in U.S. Pat. No. 4,500,589 for instance are obtained by stacking alternating sheets of metal (most preferably aluminum) and fiber-reinforced prepregs, and curing the stack under heat and pressure. These materials are increasingly used in industries such as the transportation industry, for example in ships, cars, trains, aircraft and spacecraft. They can be used as sheets and/or a reinforcing element and/or as a stiffener for (body) structures of these transports, like for aircraft for wings, fuselage and tail panels and/or other skin panels and structural elements of aircraft.
WO 2009/095381 A1 discloses a fiber-metal laminate wherein the metal volume fraction ranges between 0 and 47%.
WO 2007/145512 A1 discloses a fiber-metal laminate comprising thick metal sheets with a thickness above 1 mm. The thick metal sheets are bonded to other layers of the laminate by a fiber-reinforced composite layer having a fiber volume fraction Vf of lower than 45%.
EP 0312150 A1 and EP 0312151 A1 describe other useful fiber-metal laminates.
Although fiber-metal laminates may provide improved resistance to fatigue (in particular crack propagation) over metal alloys, in particular aluminum alloys, their behavior in a structure is still open for improvement, in particular in structures that are subject to dynamic loadings and need also high static strength and in particular high strength for mechanical joining. An important characteristic in this respect is resistance to crack growth as well as good joint strength of the structure. It would be highly desirable if the right metal sheets and fiber-reinforced composite layers could be identified in terms of their properties in view of achieving the lowest crack growth rate of the corresponding fiber-metal laminate with adequate joint strength performance.
Fiber-metal laminates of the type of those according to the invention are preferably connected to other components of a structure, and therefore may be provided with notches to accomplish the connection. Such notches provide stress concentrations that may negatively influence fatigue life. It is a further object of the invention to provide a fiber-metal laminate comprising mutually bonded fiber-reinforced composite layers and metal sheets, having an optimal notch performance in dynamic loading.
It is an object of the invention to provide a fiber-metal laminate comprising mutually bonded fiber-reinforced composite layers and metal sheets with an optimal structural response in dynamic loading, in particular with the lowest crack growth rate giving simultaneously adequate joint strength.